US-A-2005/0047657 discloses a method and apparatus for adjusting colour saturation in an image, the apparatus being shown schematically in FIG. 1 and using the HSV colour model. The colour saturation adjusting apparatus 100 of this prior art has an RGB-HSV conversion unit 110 which converts RGB values of the input RGB image signal into corresponding HSV values. A colour saturation adjusting function setting unit 120 allows a user to adjust the degree of colour saturation by inputting colour saturation adjusting values for specific hues (i.e. colours) through a colour saturation adjusting menu. It is not practical for the user to input the colour saturation values for all of the hues. Therefore, practically, the user inputs the colour saturation adjusting values for some representative hues (such as red, green, blue, yellow, cyan, and magenta) and the adjustment values for other, intermediate values are obtained by an interpolation method such as a linear interpolation. A colour saturation adjusting unit 130 then provides the colour saturation adjusting value gain corresponding to the H values and new colour saturation values OUT_S are calculated by applying the colour saturation adjusting function to the original colour saturation value S while the H and V values are fixed. The H, OUT_S, and V values obtained by adjusting the original colour saturation values S in the colour saturation adjusting unit 130 are converted into new RGB values in the HSV-RGB conversion unit 140, which outputs the new RGB values.
A problem with this prior art method is that the user inputs gain values for some representative hues only, and then the system determines the gain values for the remaining hues using a linear interpolation function. Moreover, once the gain value for a hue is set, it is used for every pixel of each video frame. In other words, the colour saturation values of each pixel are adjusted with the same gain values. The method is not adaptive to changing illumination conditions. This can lead to artificial and disturbing results in image regions with high luminance and image regions with high colour saturation.